The image below shows the flame
emission spectra of four metals and an unknown mixture of metals. By using the spectra, what metals
does the unknown mixture contain?
Flame emission spectra is another
name for atomic emission spectra. Our sample has been heated in a
flame. The light’s been collected and
passed through a prism or scattered off a diffraction grating, and the separated
frequencies of light have been collected and put together into a spectrum. We’ve been given the spectra of
samples that contain only magnesium, aluminum, copper, or lithium. The lines we see in each spectrum
are unique to that element. So, if those lines appear in our
mixture, we know our mixture contains that element.
The easiest way to approach this
question is to identify the most distinctive line, and that’s this green one over
here. The only spectrum with an identical
line that we’ve been given is that of magnesium. So we’ve accounted for that
line. But we should also see all the
other lines from the magnesium spectrum in our mixture spectrum. So what we need to do is match up
all these lines with lines in our mixture spectrum. Every single one of the lines in
the magnesium spectrum can be found in the spectrum from our unknown mixture, so we
know for certain our mixture contains magnesium.
However, there are still three
lines for which we don’t have a match. There appear to be two different
elements, lithium and aluminum, that are a match for this red line. However, the other two lines on the
aluminum spectrum match up perfectly with the other two unaccounted-for lines, while
in the spectrum of lithium there are no extra lines in the right places. If there was lithium in our sample,
we’d expect this bright orange line to show up here. But it doesn’t. Since all the lines in the aluminum
spectrum show up in our mixture spectrum, we know we also have aluminum in our
mixture. And since we’ve accounted for all
the lines in our unknown mixture spectrum, we know that our sample contains only
magnesium and aluminum.